1. Field of the Invention
The present invention relates to a coupling device for power transfer between drive and driven shafts coaxially arranged with each other for relative rotation, and more particularly to a coupling device for transmitting the power from the drive shaft to the driven shaft in response to the difference in rotational speed between the shafts.
2. Description of the Prior Art
In Japanese Utility Model Early Publication No. 59-188731, there is disclosed a viscous coupling device wherein an amount of high viscous fluid is stored to effect power transfer between drive and driven shafts when it has been sheared due to the difference in rotational speed between of the shafts. Disclosed in Japanese Patent Early Publication No. 63-240429 is a viscous coupling device which includes a friction clutch for effecting power transfer between drive and driven shafts when it has been engaged. The friction clutch is arranged to be engaged by a pressure of high viscous fluid applied thereto in response to the difference in rotational speed between the shafts. In such conventional viscous coupling devices, the power is transmitted from the drive shaft to the driven shaft and vice versa. In application to the drive system of a four-wheel drive vehicle, however, it is required to prevent transfer of the drive torque from the driven shaft to the drive shaft.